Brownian Motion
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Microscopes
Direct observation of Brownian Motion requires a microscope. Today, it is easy to purchase a microscope that has a CCD (Channel Coupled Device) attached to it. Here's two that are relatively inexpensive:
- Digital Blue QX5- http://www.playdigitalblue.com/products/qx5/info/, is a toy microscope in the price range of $70 to $80. The optics is all plastic, and very very hard to do Brownian Motion. However, it is an excelent microscope to view LEDs, circuits, etc... The light that comes with it, is a white light LED, and not too intense. You can replace it by a Superbright LED if you need more light. It comes with video capture software for windows. The Mac version is available from this web site
- Motic Microscope - student series. This is an excellent microscope for brownian motion, and costs about $350. We purchased one from The Microscope Depot. The motic web site is http://www.motic.com. It comes with video capture software.
Slides
For Brownian motion experiment you do need slides with a concave depression, they are know as well slides . This avoids the problem of having too much surface tension when you put the slide covers. The other cheap option is to use Scotch tape with a punched hole. Slides can be purchased from a lot of sources, for example electron microscopy sciences.
Latex Spheres
You can buy a sampler of latex spheres already dyed in blue from Polysicences.
Producing the solution
The easiest and cheapest way to is to use milk fat. Mix milk with water to produce a slightly cloudy mixture. Then add blue food coloring. This will produce a nice brownian motion demonstration.
The alternative is to use 1 micron latex spheres. This is very nice and allows you to do measurements.
Alternative Techniques
Alternatively, if you don't want to do the direct measurement, you can choose to do a laser scattering experiment. Paul Doherty from the Exploratorium has a nice web site explaining how to do this.
Data Analysis
With the microscope and software listed above (or another microscope with video capture capability), some data analysis can be done. A simple program called DataPoint (go here to download) allows you to track particles and records the x and y position as well as the time. It uses .avi format movies, and divides the movie into 15 frames per second. Analyzing a one minute movie can be quite tedious using all 15 frames per second, so you might think about only recording one or two points every second.
By pasting the data file it creates into excel, you can plot the motion and come up with a nice plot showing the random motion.
In Einstein's paper on Brownian motion in 1905, he predicted that the average R^2 would have a linear relationship with time. Using either R^2 = (RT/3aηN)t or R^2 = (4kT/6πηN)t you can get a nice plot (using the time from the data file from DataPoint) of R^2 vs. time. (R=universal gas constant; T=temperature; a=particle radius; η=viscosity of liquid; N=Avagadro's number; k=Boltzmann's constant; t=time)
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